1. Field of the Invention
This invention relates to a power steering system of the automobile.
2. Description of the Prior Art
One known type of power steering system of the automobile comprises an input shaft coupled to a steering wheel, a torsion bar for transmitting rotation of the input shaft to an output shaft, a power cylinder coupled to the output shaft, an oil path switching valve for switching oil paths to the power cylinder in accordance with the difference of turning angle between the input shaft and the output shaft, a high pressure oil path for supplying working oil delivered from an oil pump to the power cylinder via the oil path switching valve, a main orifice provided in the midway of the high pressure oil path, a low pressure oil path for returning the working oil from the power cylinder to an oil tank via the oil path switching valve, a reaction piston for exerting a restraining force between the input shaft and the output shaft to limit the difference of turning angle therebetween, a control oil path extending from the midway of the high pressure oil path to the reaction piston, a pressure control valve for controlling oil pressure in the control oil path extending to the reaction piston to a level not higher than a predetermined highest pressure, a pair of parallel oil paths branched from the midway of the control oil path between the pressure control valve and the reaction piston, a second orifice provided in one of the parallel oil paths, a flow rate control valve for selecting either one of the parallel oil paths to control the flow rate of the working oil flowing therethrough to a level corresponding to a car speed, a first orifice for generating in the downstream side oil path of the flow rate control valve a pilot pressure corresponding to the flow rate of the working oil, a pilot oil path for supplying the pilot pressure to the pressure control valve, a bypass path for bypassing the upstream side high pressure oil path and downstream side high pressure oil path of the main orifice and a change-over valve for blocking the bypass path only when the oil pressure on the downstream side of the pressure control valve becomes lower than a predetermined lowest pressure to increase the oil pressure in the whole control oil path (see, for example, Japanese Patent Application No. 58-86598: Laid-Open No. 59-213564).
The foregoing power steering system of the automobile must include a number of high precision components, such as the main orifice provided in the midway of the high pressure oil path, the parallel oil paths of a pair branched from the midway of the control oil path between the pressure control valve and the reaction piston, the second orifice provided in one of the parallel oil paths, the flow rate control valve for selecting either one of the parallel oil paths to control the flow rate of the working oil flowing therethrough to a level corresponding to a car speed, the bypass path for bypassing the upstream side high pressure oil path and downstream side high pressure oil path of the main orifice, and the changeover valve for blocking the bypass path only when the oil pressure on the downstream side of the pressure control valve becomes lower than a predetermined lowest pressure to increase the oil pressure in the whole control oil path, so that its manufacturing cost is increased.
Further, in this known power steering system, the pilot pressure generated on the downstream side of the flow rate control valve by means of the first orifice is led via the pilot oil path to the pressure control valve and caused to actuate the pressure control valve; thus, it is necessary to cause the working oil of a certain flow rate to flow from the oil pump to the control oil path even in the standing state. Therefore, at the time of stationary steering in a low speed engine rotating state where the working oil of high pressure is needed, a part of the working oil (of a constant flow rate as mentioned above) flowing from the oil pump to the power cylinder is consumed in a control circuit; thus, the capacity of the oil pump must be increased beyond a standard capacity by the amount of consumption. In addition, at the time of stationary steering, the working oil of high pressure flowing into the control circuit is controlled by the pressure control valve to a lowest pressure and led to the reaction piston section, thereby resulting in the defect that a sound of oil flow is generated in the control section of the pressure control valve.